The present invention relates generally to wide dynamic range amplifiers and more particularly to a transimpedance amplifier having error correction circuitry for reducing non-ideal amplifier characteristics in the output signal of the amplifier.
Transimpedance amplifiers are used in many electronic applications for converting an input current signal to a corresponding proportional voltage signal. The voltage across the feedback element of the transimpedance amplifier depends on the impedance of the feedback element and the current through the element according to Ohm's law. If the voltage at one end of the feedback element remains at 0 volts, then the voltage at the other end is proportional to the current through the element. The assumption made for most transimpedance amplifier designs is that the input summing junction remains at 0 volts. For more accurate designs, gains are adjusted by a fixed amount to correct for the finite gain of the amplifier. Other errors are less predictable and cannot normally be corrected for, such as: amplifier gain changes with time, temperature, and signal level; offsets created by temperature changes in transistors as the signal level changes; offset created by charge stored in circuit capacitance when overloads occur; and slew rate errors which depend on the signal levels and circuit clamp techniques. Each of these errors can be significant in measurement instruments, such as an optical time domain reflectometer, OTDR, due to the requirements of wide bandwidth, high dynamic range, sensitivity to low-level signals, and accurate measurement of light level versus time.
In an OTDR, light pulses are launched into a fiber under test. During the intervals between the light pulses, return reflected light, in the form of Rayleigh backscatter and high amplitude reflections, is converted to a current signal, amplified, digitized, and stored for further processing. The return reflected light signal has an optical dynamic range as large as 100 dB which corresponds to an electrical dynamic range of 200 dB. In addition, the Rayleigh backscatter signal decreases exponentially with distance generating increasingly weak return backscatter signals from the distant portions of the fiber. A photosensitive device, such as an avalanche photodiode or the like, converts the return reflected light into a current signal. The corresponding current signal is coupled to a transimpedance amplifier, which converts the current signal into a corresponding proportional voltage signal. The voltage output of the transimpedance amplifier, which ranges down to the nanovolt region, is further amplified and coupled to an analog-to-digital converter, which converts the voltage signal into corresponding digital values. The digital values are stored in a memory device, such as RAM, in the OTDR and are subsequently retrieved for further processing and display.
For an ideal transimpedance amplifier, the voltage at the input of the amplifier is always zero. With this assumption, V.sub.out equals the magnitude of the input current multiplied by the impedance of the feedback element. If the impedance and characteristics of the feedback element are known, then the photodetector current is represented by the voltage at V.sub.out. The feedback element may be linear as in the case of a resistor or nonlinear as in the case of a diode or the like. This is the technique used in most OTDRs. The quality of V.sub.out representing the photodetector current suffers due to the non-ideal characteristics of the amplifier and associated amplifier components. The first non-ideal characteristic of the amplifier, which affects the quality of V.sub.out is amplifier gain. The gain of the amplifier is lower than ideal, and the gain varies with the signal level applied. With low gain, the voltage at the input of the amplifier will change with signal level. As the gain varies with signal level, V.sub.out will be nonlinear. The second non-ideal characteristic of the amplifier which affects the quality of V.sub.out is offset voltages created due to thermal effects. As the signal of the amplifier changes, the components which provide the gain will change temperature, creating offsets which depend on the temperature at any given instant. These offsets combine with the signal and reduce the quality of V.sub.out, creating errors often called nonlinearities, undershoot, and thermal tail. Since the voltage on the input node does not remain at zero volts, currents may be produced in the associated amplifier components, such as input clamping diodes, circuit capacitances, and the like. These currents subtract from the input current being coupled to the feedback element producing additional output errors.
U.S. Pat. No. 5,123,732 describes a current-voltage converter for use in an OTDR having an transimpedance amplifier with an input port, an output terminal and a resistor coupled between the input port and the output terminal. A voltage clipping means, in the form of a clipping diode, is coupled between the input port and a fixed potential to prevent saturation of the transimpedance amplifier due to high current input signals caused by high amplitude return signals from a fiber under test. The potential on the clipping diode is such that capacitive effects of the diode are reduced. However, other errors, such as offsets created by temperature changes as the input signal level changes, changes in the gain of the amplifier with the signal level applied, and the like are not reduced or eliminated with this circuit.
What is needed is a wide dynamic range amplifier that accurately produces an output signal representative of an input signal without errors associated with the non-ideal characteristics of such amplifiers. Further, the wide dynamic range amplifier should be capable of correcting other errors caused by circuit components, such as clamping diodes, circuit capacitances, and the like.